1. Field of the Invention
The present invention refers to a method for the quantitative and/or qualitative characterization of substances contained in a gaseous carrier medium, e.g. for detecting the nature and/or the amount of small solid particles contained in a carrier gas, or for detecting the presence of an unknown gas in a carrier gas etc. Particularly the invention refers to a method of the aforementioned kind in which small solid particles suspended in a carrier gas and unknown gases or gas traces possibly contained in the carrier gas are detected and characterized by admixing an agent to the carrier gas to be examined or by effecting a photochemical reaction in the carrier gas to be examined.
2. Prior Art The well known photoelectric effect, being a surface sensitive method, is very well suited to furnish evidence of even the slightest chemical modification of the surface of small solid particles. Particularly the photoelectric yeild (i.e. the rate of electron emission divided by the intensity of the radiation) is sensitively changed even by adsorbant layers on the surface of the particles which have but a monoatomic or monomolecular thickness. A similar effect may be observed if the surface of the particles is modified by a chemical reaction. The photoemission from small solid particles suspended in a gaseous carrier may be used to detect and characterize a chemical modification of the surface of these particles. Usually it is understood that small solid particles suspended in a gaseous carrier have a diameter in a range between 1 nm and 10 .mu.m. They have the property to remain suspended in the gaseous carrier, depending of their size for a longer or shorter period. Such a suspension, i.e. a carrier gas containing suspended small solid particles, is known as an aerosol.
Upon the photoemission from suspended particles contained in a gaseous suspension, the particles having a positive charge are left. The electrons diffuse in the gas and usually form negative ions. Evidence of such microscopic charge separation may be proved according to different methods. One of the known methods is disclosed in reference (1) and is designated as measurement of the aerosol photo conductance. Thereby the conductance of the gas within an electric alternating current field is measured. Other known methods are based on the principle that a different diffusion rate or mobility of the photoelectrons or of the ions generated therefrom as compared to the particles results in a macroscopic charge separation, so that the positive and/or the negative charges may be determined separately. According to reference (2), the electric current generated by the photoelectrically positively charged, continuously flowing particles is measured, after the photoelectrons have been removed from the aerosol by diffusion and drift in the electric field.